3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard with e.g. increased capacity and higher data rates towards the fourth generation of mobile telecommunication networks. Hence, the LTE specifications provide downlink peak rates up to 300 Mbps, an uplink of up to 75 Mbit/s and radio access network round-trip times of less than 10 ms. In addition, LTE supports scalable carrier bandwidths from 20 MHz down to 1.4 MHz and supports both FDD (Frequency Division Duplex) and TDD (Time Division Duplex).
LTE uses OFDM (Orthogonal Frequency Division Multiplex) for the downlink, wherein several subcarriers are used instead of one carrier. In the time domain there is a radio frame that is 10 ms long and consists of 10 subframes of 1 ms each. Every subframe consists of two slots where each slot is 0.5 ms. The subcarrier spacing in the frequency domain is 15 kHz. 12 of these subcarriers (per slot) is called a resource block.
LTE-Advanced implies further improvements of the LTE systems where one of the most important improvement areas in LTE-Advanced is the increase of data rates available for users at the cell edge. A promising technique to achieve this goal is the deployment of relays. However, the use of relays implies that the signal processing of the relays are affected by an interference referred to as self interference.
FIG. 1, which is further explained below, discloses a relay structure with self Interference cancellation (SelfIC). The relay structure is an equivalent complex baseband structure, i.e. any radio-related functionality is excluded in this description. Each of the blocks (r, w1,a,w2,t,bc,b1) implements a filter and in case of a MIMO enabled relay even matrix of filters, as disclosed in FIG. 2 for a 2×2 MIMO configuration. In case that there is no coupling between the different MIMO paths the diagonal elements h12(n) and h21(n) vanish. In case of a SISO element only h11(n) remains and is denoted simply h(n).
Mathematically the entity of FIG. 2 can be described by the matrix
                                          h            ⁡                          (              n              )                                =                      (                                                                                                      h                      11                                        ⁡                                          (                      n                      )                                                                                                                                  h                      12                                        ⁡                                          (                      n                      )                                                                                                                                                              h                      21                                        ⁡                                          (                      n                      )                                                                                                                                  h                      22                                        ⁡                                          (                      n                      )                                                                                            )                          ,                            (        1        )            which elements contain impulse responses of the different paths. The output-input relation of this entity can be described as
                              y          ⁡                      (            n            )                          =                                            {                              h                *                x                            )                        ⁢                          (              n              )                                =                                                    (                                                                                                    h                        11                                                                                                            h                        12                                                                                                                                                h                        21                                                                                                            h                        22                                                                                            )                            *                              (                                                                                                    x                        1                                                                                                                                                x                        ⁢                                                                                                  ⁢                        2                                                                                            )                            ⁢                              (                n                )                                      =                                          (                                                                                                                                                          {                                                                                          h                                11                                                            *                                                              x                                1                                                                                      }                                                    ⁢                                                      (                            n                            )                                                                          +                                                                              {                                                                                          h                                12                                                            *                                                              x                                2                                                                                      }                                                    ⁢                                                      (                            n                            )                                                                                                                                                                                                                                                        {                                                                                          h                                21                                                            *                                                              x                                1                                                                                      }                                                    ⁢                                                      (                            n                            )                                                                          +                                                                              {                                                                                          h                                22                                                            *                                                              x                                2                                                                                      }                                                    ⁢                                                      (                            n                            )                                                                                                                                              )                            .                                                          (        2        )            In the following the functionality of each entity in FIG. 1 will be described. x(n) is the signal entering the relay and y(n) is the output signal of the relay. Depending on the antenna setup of the relay x(n) and y(n) are either scalars or vectors.
The block r(n) 102 describes any non-ideal behavior that the signal is subjected to when entering the relay, e.g. antenna coupling, radio frequency imperfections, imperfect analog digital converter, etc.
The block t(n) 106 describes similar effects at the output of the relay. Furthermore, t(n) 106 absorbs any analogue amplification functionality. These two blocks vanish in the ideal case, i.e. r(n)=t(n)=δ(n), with δ(n) denoting the Kronecker delta function. (δ(n) is 1 for n=0 and 0 otherwise). For 2×2 MIMO relays this implies r11(n)=r22(n)=t11(n)=t22(n)=δ(n) and r12(n)=r21(n)=t12(n)=t21=0.
The block w1(n) 103 describes matched filtering—in case of a SISO relay—or receiver beamforming for MIMO relays. In the latter case the filter w1(n) 103 is replaced by a matrix of filters according to FIG. 2. Depending on the actual impulse response(s) of w1(n) 103 other functions can be implemented as well.
The block w2(n) 105 describes accordingly transmitter beamforming or other desired signal processing applied to the relay output signal.
The core-relay functions will be performed in block a(n) 104. In case of SISO this function is either a(n)=δ(n) in which case a non-frequency selective relay is described or a(n) is the impulse response of a more general filter, in which case a(n) 104 describes a frequency-selective relay. For higher layer relays a(n) 104 can even include decoding and re-encoding functionality.
In case of MIMO the block a(n) 104 is replaced by a matrix of signal processing functions.
The undesired self interference is described by block b1(n) 108 and also here b1(n) 108 is either described by a single filter or by a matrix of filters. Since interference occurs between the transmit antenna output and the receive antenna input the interference path originates after t(n) 108 and terminates before r(n) 102. The entity b1(n) 108 is actually a complex baseband representation, i.e. effects of modulation and demodulation are included in b1(n).
In order to mitigate this interference a Self Interference Canceller (SelfIC) bC(n) 107 is used. In the ideal case—when the interference canceller is able to completely cancel the interference—only the forward signal path r(n)→w1(n)→a(n)→w2(n)→t(n) remains. Please note that for the interference canceller to work a delay larger than 0 is needed in the loop w1(n)→a(n)→w2(n)→bC(n).
By slightly redrawing the block diagram of FIG. 1, essentially the interference feedback is moved inside r(n) and t(n) and the effect of these entities is considered by modifying the feedback function b1(n) 108 to {t*b*Ir}(n) 108′ which is illustrated in FIG. 3. It is important to note that this block diagram is equivalent to the block diagram shown in FIG. 1.
In Orthogonal Frequency Division Multiplex (OFDM) the serial data stream is converted into N parallel data streams and each of these data streams—which have now an N-times lower symbol rate—is used to modulate sinusoidal carriers. Each of the individual sinusoidal carriers is called a subcarrier.
The modulation with sinusoidal carriers can be very efficiently implemented in the frequency domain by a Discrete Fourier Transform (DFT). If the data to be transmitted are collected in the frequency-domain vector X the transmitted time-domain signal becomesx(n)=idft{X}(n).  (3)If this signal is transmitted across a channel h(n) that performs a circular convolution the output signal in frequency domain becomesY(k)=H(k)·X(k),  (4)with H(k) being the N-point DFT of h(n) evaluated at subcarrier k. As it can be seen, the data received on subcarrier k only depends on the data transmitted on subcarrier k which makes the receiver implementation very simple. The circular convolution between an input signal and a channel is the linear convolution between the channel and the periodic extension of the original input signal.
However, a physical radio channel does not perform a circular convolution but instead a linear convolution on its input signal due to the fact that the input signal is not periodic. In order to transform this linear convolution performed by the channel into a circular convolution—at least within a certain time frame—(i.e. to make the filter believe that it works on a periodic signal) each OFDM symbol is prefixed by a Cyclic Prefix (CP) in order to make the signal appear periodic over certain time frame, wherein the CP is a copy of the last part of the OFDM symbol. After the complete memory of the channel is excited with the first part of the input signal, the input signal appears periodic to the channel and the linear convolution becomes identical to the circular convolution. In an OFDM system that deploys a CP the output signal therefore becomes the circular convolution after the complete channel memory has been excited by the first part of the input signal.
FIG. 4 depicts this graphically. From time −P+Lh−1 and onwards up to N−1 the output signal y(n) is the circular convolution between h(n) and x(n). Applying an N-point DFT to any N consecutive samples y(n) starting between −P+Lh−1 and 0 yields H(k)·X(k)·P(k), with P(k) being a linear phase that stems from a starting position of the DFT window not equal to zero. Depending on the length of the channel impulse response relative to the CP length multiple position exist all yielding H(k)·X(k)·P(k), to be more specific P−Lh+2 such positions exist. In case the impulse response is longer than the cyclic prefix no such position exists, i.e. the liner convolution performed by the channel will never be equal to the circular convolution. The relationship H(k)·X(k)·P(k) for the frequency-domain representation of the convolution is no longer valid and data transmitted on one subcarrier interfere with data received on another subcarrier.